Force sensitive scales, i.e., scales which are capable of measuring the applied force due to gravity of an object placed upon the scale, have numerous applications. One such application, for example, is on lift trucks, or forklifts, to weigh objects which are lifted and transported by the forks of the lift truck as they are being transported. Lift truck scales preferably do not require any specific operation by an operator of the lift truck other than positioning the truck and lifting the objects. As a result, the weighing operation is very efficient and non-intrusive.
One example of a lift truck scale which is particularly useful in the art is the subject of U.S. Pat. No. 4,421,186 to Bradley, which is incorporated by reference herein. Generally, Bradley discloses a lift truck scale including a vertically oriented plate that is mounted to the existing crossbars disposed on the front of a standard lift truck. The plate is mounted through four load-bearing load cells arranged into two laterally-spaced vertical columns, which serve as the sole mechanical connection between the scale and the existing crossbars. The conventional forks used with the lift truck are hung off of the vertical plate in the same manner as they would the crossbars, whereby a load present on the forks is transferred across the load cells by the vertical plate.
Each load cell in the Bradley design includes four strain gauges connected in a wheatstone bridge circuit to reject many non-load force effects. Moreover, the outputs of the load cells are summed to provide the total weight reading for the scale.
The Bradley lift truck scale is capable of rejecting many non-load force effects (i.e., forces and moments which are not found along the primary force vector that is being measured). The Bradley scale is therefore relatively accurate, typically having an accuracy of about 1%. Bradley also has the advantage that it is easy to install on conventional lift trucks, thereby making it relatively cost effective. Also, the Bradley scale has a low profile which does not significantly decrease the overall carrying capacity of the lift truck.
On the other hand, the Bradley scale has been found to exhibit some degree of position sensitivity despite efforts to reduce the variations that occur as a result of different positioning of objects on the forks. These effects are most prevalent in the fore/aft direction (i.e., along the longitudinal axes of the forks), primarily due to "end effects" applied to the load cells at some positions. The end effects are maximized when an object is located near the tips of the forks, as a relative large horizontal separation exists between the load cells and the object in this position, resulting in non-load forces that are comparable in magnitude to the load forces along the primary force vector for the scale.
By "end effects", we mean primarily end loads and end moments, as well as other forces, that result in forces applied to the load cells that are not along the force vector of the desired forces to be measured (i.e., those representing the weight of an object). These end effects are mostly rejected in scales such as Bradley by the mechanical structure of the load cells and the strain gauge bridge circuits. However, it has been found that, particularly in lift truck scales where end effects are relatively great, these end and other effects are not purely rejected. This results in "residual rejected effects" being present in the output signal of each load cell, thereby limiting the accuracy of the scale and making the scale sensitive to the position of objects thereon.
It has also been found that the vertical arrangement of pairs of loads cells in the Bradley scale may introduce some degree of frictional effects (primarily creep and hysteresis) into the outputs of the load cells due to slippage in the mechanical couplings between the plate and the load cells, as well as due to relaxation in the structures of the plate and the load cells alike. In theory, a pair of vertically oriented load cells coupled in parallel should perfectly share a load such that the summed outputs of the cells remain constant even if the load is shifted between the load cells. In practice, however, it has been found that these frictional effects may limit this load sharing effect and introduce lag errors into the system, also reducing the accuracy of the scale.
Therefore, a substantial need exists for a scale and load cells for use therewith which more perfectly reject residual rejected effects and frictional effects from the output of the load cells. In addition, a need exists for a scale which is of greater accuracy and reduced sensitivity to the position of objects on the scale.